Real-time phase synchronization of a remote receiver with a measurement instrument

ABSTRACT

A measurement instrument for measuring electrical characteristics of a device under test (DUT) includes a synchronization signal generator for generating a synchronization signal transmittable to a receiver, a voltage controlled phase shifter (VCPS) connected with the synchronization signal generator and a phase-to-voltage converter configured to drive the VCPS. The synchronization signal is transmitted via a duplexer configured to transmit the synchronization signal from the measurement instrument to the receiver via a fiber optic cable and retransmit the received synchronization signal from the receiver to the measurement instrument via the fiber optic cable. The phase-to-voltage converter receives as inputs the synchronization signal input to the VCPS, the synchronization signal output from the VCPS and the retransmitted synchronization signal received at the measurement instrument from the receiver. An output of the phase-to-voltage converter is provided as input to the VCPS.

CLAIM OF PRIORITY

This application claims the benefit of priority to U.S. ProvisionalApplication titled “SYNCHRONIZATION OF A REMOTE WIDE BAND RECEIVER USINGA NARROW BAND LOW FREQUENCY SIGNAL OVER A FIBER OPTIC CABLE”,Application No. 62/255,290, filed Nov. 13, 2015 and U.S. ProvisionalApplication titled “EXACT PHASE SYNCHRONIZATION OF A REMOTE RECEIVEROVER A SINGLE DUPLEXED FIBER OPTIC CABLE”, Application No. 62/255,284,filed Nov. 13, 2015, which applications are herein incorporated byreference.

TECHNICAL FIELD

The present invention relates generally to methods of measuring anelectrical response of a device under test (DUT) and instruments thatmeasure an electrical response of a DUT to test signals.

BACKGROUND

Measuring an electrical response to test signals of a device under test(DUT), such as a component or components of a telecommunication(telecom) network, can provide information about how the component orcomponents of the DUT affect signals transmitted through the DUT. A DUTcan affect both the amplitude and the phase of a signal. Powermeasurements can communicate how a DUT attenuates different frequencycomponents of a signal. Phase measurements can communicate how a DUTaffects the phase of different frequency components of a signal. Forexample, a phase measurement of the response of a DUT to a test signalcan determine the group delay for each of the frequency components ofthe test signal, and can be used to determine how linear the response ofthe DUT is to the test signal. Phase measurements can also help trackthe path of a test signal or determine causality, e.g. locating a sourceof passive intermodulation (PIM) within a DUT.

Phase measurements using a transmitter and a receiver separate from eachother requires the transmitter and the receiver to be synchronized, forexample using a synchronization signal transmitted from the transmitterto the receiver. However, if the distance of a transmission path throughthe DUT requires the transmitter and the receiver to be separated by alarge distance, phase information about the DUT can be difficult toobtain using a synchronization signal transmitted between thetransmitter and the receiver based on methods and instruments inaccordance with the prior art because a large signal path introducesphase to the synchronization signal and attenuates the synchronizationsignal. Embodiments of the present invention are intended to addressthis difficulty.

SUMMARY

In accordance with an embodiment, a measurement instrument for measuringelectrical characteristics of a device under test (DUT) includes asynchronization signal generator for generating a synchronization signaltransmittable to a receiver, a voltage controlled phase shifter (VCPS)connected with the synchronization signal generator and aphase-to-voltage converter configured to drive the VCPS. Thesynchronization signal is transmitted via a duplexer configured totransmit the synchronization signal from the measurement instrument tothe receiver via a fiber optic cable and retransmit the receivedsynchronization signal from the receiver to the measurement instrumentvia the fiber optic cable. The phase-to-voltage converter receives asinputs the synchronization signal input to the VCPS, the synchronizationsignal output from the VCPS and the retransmitted synchronization signalreceived at the measurement instrument from the receiver. An output ofthe phase-to-voltage converter is provided as input to the VCPS.

In accordance with an embodiment, a system for measuring electricalcharacteristics of a device under test (DUT) includes a measurementinstrument adapted to be connected with the DUT for transmitting testssignals to the DUT and a receiver. The measurement instrument includes asynchronization signal generator, comprises a voltage controlled phaseshifter (VCPS) connected with the synchronization signal generator and aphase-to-voltage converter configured to drive the VCPS. The receiveradapted to be connected with the DUT and arranged remote from themeasurement instrument to receive the test signals transmitted by themeasurement instrument to the DUT. A duplexer is configured to transmita synchronization signal from the measurement instrument to the receiverand retransmit the received synchronization signal from the receiver tothe measurement instrument. The phase-to-voltage converter receives asinputs the synchronization signal input to the VCPS, the synchronizationsignal output from the VCPS and the retransmitted synchronization signalreceived at the measurement instrument from the receiver. The output ofthe phase-to-voltage converter is provided as input to the VCPS.

In accordance with an embodiment, a method for measuring electricalcharacteristics of a device under test (DUT) based on transmitted testsignals includes using a measurement instrument comprising asynchronization signal generator, a voltage controlled phase shifter(VCPS) connected with the synchronization signal generator and aphase-to-voltage converter configured to drive the VCPS. A test port ofthe measurement instrument is connected to the DUT. A test port of areceiver is also connected to the DUT such that the receiver receivestest signals transmitted by the measurement instrument and through theDUT. A synchronization port of the measurement instrument is connectedto a synchronization port of the receiver via a fiber optic cable and aduplexer such that the receiver receives the synchronization signal.

The phase-to-voltage converter receives as inputs the synchronizationsignal input to the VCPS, the synchronization signal output from theVCPS and the retransmitted synchronization signal received at themeasurement instrument from the receiver. An output of thephase-to-voltage converter is provided as input to the VCPS. The phaseof the synchronization signal at the synchronization port of themeasurement instrument is then synchronized to the phase of thesynchronization signal at the synchronization port of the receiver.Electrical characteristics of the DUT based on the transmitted testsignals can then be measured, including phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the present invention are explained with the help ofthe attached drawings in which:

FIG. 1 is a circuit diagram of a measurement instrument and a remotereceiver connected through a device under test (DUT) and synchronized,in accordance with the prior art.

FIG. 2 is a circuit diagram of a measurement instrument and a remotereceiver connected through a DUT and synchronized using a single,digital synchronization signal, in accordance with an embodiment.

FIG. 3 is a circuit diagram of a measurement instrument and a remotereceiver connected through a DUT and synchronized using a single, analogsynchronization signal, in accordance with an embodiment.

FIG. 4 is a circuit diagram of a locking loop for locking a localoscillator (LO) signal to a digital synchronization signal, inaccordance with an embodiment.

FIG. 5 is a circuit diagram of a network analyzer and a remote receiverconnected through a DUT and synchronized using a single, digitalsynchronization signal, in accordance with an embodiment.

FIG. 6 is a circuit diagram of a measurement instrument providing asynchronization signal to a remote receiver through a connectionincluding a duplexer, the circuit diagram being configured to measure alength of the connection, in accordance with an embodiment.

FIG. 7 is a circuit diagram of an implementation of a portion of thecircuit diagram of FIG. 6, in accordance with an embodiment.

FIG. 8 is a circuit diagram of a measurement instrument configured tosynchronize a phase of a signal received at a remote receiver with aphase of a signal transmitted from the measurement instrument through aconnection including a duplexer, in accordance with an embodiment.

FIG. 9 illustrates a duplexer for both transmitting and receivingsignals along a fiber optic cable for use with embodiments of thepresent invention.

FIG. 10 illustrates a pair of vector network analyzers (VNAs)synchronized by a fiber optic cable, in accordance with an embodiment.

FIG. 11 illustrates a pair of VNAs synchronized by a fiber optic cableand connected at test ports to cables extending through bulkhead wallsof a ship, in accordance with an embodiment.

FIG. 12 illustrates a pair of VNAs synchronized by a fiber optic cableconnected by way of fiber extenders multiplexed to DUTs comprising aplurality of antennas acting as DUTs, in accordance with an embodiment.

DETAILED DESCRIPTION

The following description is of the best modes presently contemplatedfor practicing various embodiments of the present invention. Thedescription is not to be taken in a limiting sense but is made merelyfor the purpose of describing the general principles of the invention.The scope of the invention should be ascertained with reference to theclaims. In the description of the invention that follows, like numeralsor reference designators will be used to refer to like parts or elementsthroughout.

In the following description, numerous specific details are set forth toprovide a thorough description of the invention. However, it will beapparent to those skilled in the art that the invention may be practicedwithout these specific details. In other instances, well-known featureshave not been described in detail so as not to obscure the invention.

Referring to FIG. 1, a vector network analyzer (VNA) is shown connectedwith a device under test (DUT), which is also connected with a remotereceiver to form a two-port network for which scattering (S-) parameterscan be measured. A local oscillator (LO) signal and an analog-to-digital(A/D) clock signal are separately generated for use in makingS-parameter measurements. The LO signal is used to downconvert radiofrequency (RF) signals to intermediate frequency (IF) signals forprocessing by a receiver. The A/D clock signal is used to sample thedownconverted IF signals. Both the LO and A/D clock signals aresynchronized to a reference signal (REF) generated by a referenceoscillator. An RF source generates test signals for transmission to theDUT and is also synchronized to the reference signal.

As shown, incident power waves (INC) are coupled by the VNA to a mixerfor downconversion and then sampled to obtain an “a1” measurement.Likewise, reflected power waves (REFL) are coupled by the VNA to anothermixer for downconversion and then sampled to obtain a “b1” measurement.From the a1 and b1 measurements, the magnitude and phase of the forwardreflection (S11) parameter can be determined. Transmitted power waves(TRANS) are coupled by the remote receiver to a mixer for downconversionand then sampled to obtain a “b2” measurement. The b2 measurement can becommunicated to the VNA via data transmission for use by the VNA incalculating a forward transmission (S21) parameter. The remote receiveris synchronized to the VNA by using LO and A/D clock signals that aresynchronized to the LO and A/D clock signals of the VNA. These signalsare needed to retain the phase component of the received test signal tothe reference signal at the source.

Existing techniques for synchronizing two remote ports of a measurementinstrument for measuring electrical response, such as a VNA, can includecabling ports for the LO signal, or multiple LOs if multiple downconversions are used, and the A/D clock signal to ports of a remotereceiver. The LO and A/D clock signals of the VNA are transmitted to theremote receiver. The LO signal of the VNA can then be used todownconvert the test signal and the A/D clock signal of the VNA can beused to sample the downconverted test signal. If the phase component isretained, the magnitude and phase of the forward transmission (S21)parameter can be determined from the b2 and a1 measurements. If thephase component is not retained, only the magnitude of the S21 parametercan be determined. If the remote receiver is a VNA, the master/slaveroles of the two measurement instruments can be reversed to similarlyobtain measurements for the reverse reflection (S22) parameter and thereverse transmission (S12) parameter.

Typically, LO and A/D clock signals are transmitted between ameasurement instrument and a remote receiver via coaxial cable. However,practical considerations prevent signals in the microwave region frombeing extended beyond several meters (m). Phase shifts from the point oftransmission at the measurement instrument to the point of reception atthe remote receiver are introduced to the LO and A/D clock signals andscale linearly with frequency for a given length. Because of this phaseshift, for use cases where the VNA and remote receiver of FIG. 1 must bearranged at some unknown distance greater than some threshold, e.g.further than 20 m for signals having frequencies of 50 MHz or higher,the arrangement of FIG. 1 is not usable for determining the phase of theS21 and S12 parameters unless the phase shift is accounted for.

Measurement instruments, such as the VNA and remote receiver, can becalibrated to account for phase shift by directly connecting themeasurement instruments and performing such a calibration and thereafterconnecting the measurement instruments through the DUT. However, thismay not be practical depending on the use case. Further, the phase shiftcan be calculated and therefore can be compensated for in phasemeasurements for the DUT. However, the length and propagation velocityof the transmission medium must be known. If the length of thetransmission path is unknown, the phase shift cannot be calculated.

In addition to phase shift, signals transmitted over coaxial cable areattenuated. Coaxial cable signal attenuation increases with both higherfrequency and longer transmission length. For example, RG-58 is a typeof coaxial cable used for low-power signal and RF connections whichattenuates signals at 10.8 decibels (dB) per 100 m at 50 MHz increasingto 70.5 dB per 100 m at 1 GHz. The transmission length can be increasedthrough the use of repeaters or amplifiers, but a further unknown phasecomponent sensitive to environment is thereby introduced.

Embodiments of the present invention relate to transmitter and receiverarchitectures that use a narrow band, low frequency signal from a RFgenerator source port provided to a remote receiver port to retaindown-converted as well as A/D clocking while retaining absolute phaseinformation over the complete test frequency operational range.

FIG. 2 is a circuit diagram of a measurement instrument and a remotereceiver connected through a DUT and synchronized using a single, lowfrequency digital signal that is transmitted from the measurementinstrument to the remote receiver, in accordance with an embodiment. Thelength of the coax cable for synchronizing the measurement instrumentand the remote receiver of FIG. 1 is limited based at least on the 50MHz A/D clock signal. The relatively lower frequency (e.g., 10 MHz)synchronization signal generated and transmitted in accordance with anembodiment can travel over a longer transmission path without beingunusably attenuated by coaxial cable or twisted pair transmission lines,for example.

As shown, the synchronization signal is the lowest common denominator ofthe measurement instrument and is generated from the IF signal using aphase-locked loop (PLL) to maintain synchronization between the LO andthe A/D clock signal generated by a frequency multiplier. A frequency of10 MHz is merely exemplary, but is usefully low as a synchronizationsignal to allow greater lengths of transmission line runs without beingso low as to introduce an unreasonable amount of phase noise.

The IF signal generated at a mixer from the LO signal and RF signal isconverted to a digital signal by a limiter. The digital signal isprovided as input to the PLL and to the frequency multiplier thatmultiplies the signal for synchronously sampling the IF signal based onthe amount of sampling performed for measurements (e.g., M=5 for 50 MHzsampling). The digital signal is also provided as the synchronizationsignal to the remote receiver. The digital signal is output as thesynchronization signal directly from a synchronization output port (SYNCOUT) to the coax cable or twisted pair transmission line.

The remote receiver uses the synchronization signal received at asynchronization input port (SYNC IN) to synchronize a LO signal and anA/D clock signal generated from the synchronization signal by afrequency multiplier. The LO signal is synchronized using a PLL and downconverts a received test signal to an IF signal that is then sampled toobtain a transmission measurement. A b2 measurement can be communicatedto the VNA from the remote receiver via data transmission along thetransmission path of the synchronization signal (i.e., from SYNC IN toSYNC OUT) for use by the VNA in calculating a forward transmission (S21)parameter.

In a further embodiment, a system and method for synchronizing ameasurement instrument and remote receiver can use fiber optic cable forthe transmission of the synchronization signal. Attenuation of thesignal can be dramatically reduced relative to a coaxial cable ortwisted pair transmission line. Attenuation loss can be as low as 0.2dB/km in optical fiber cables, allowing transmission over long distanceswithout the need for repeaters. In such an embodiment, thesynchronization signal is converted from a digital signal to an opticalsignal and transmitted to the remote receiver whereupon the opticalsignal is converted back to a digital signal for use in synchronizingthe LO and A/D clock signals of the remote receiver. The remote receiverlikewise relies on a PLL to synchronize the LO and A/D clock signals.

The synchronization between the measurement instrument and remotereceiver will also suffer from phase shift from the point oftransmission to the point of reception, but as noted above, the distancebetween the measurement instrument and remote receiver is less limiteddue to attenuation relative to the prior art and synchronization canstill be achieved through calibration or calculation given a knownlength of the transmission path.

FIG. 3 is a circuit diagram of a measurement instrument and a remotereceiver connected through a DUT and synchronized using a single, lowfrequency analog signal that is transmitted from the measurementinstrument to the remote receiver, in accordance with an embodiment. Aswith FIG. 2, the relatively lower frequency (e.g., 10 MHz)synchronization signal generated and transmitted in accordance with anembodiment can travel over a longer transmission path without beingunusably attenuated by coaxial cable or twisted pair transmission lines,for example. However, unlike FIG. 2 the measurement instrument transmitsan analog signal rather than a digital signal.

As shown, the synchronization signal is the lowest common denominator ofthe measurement instrument and is generated from the IF signal using aPLL to maintain synchronization between the LO and the A/D clock signalgenerated by a frequency multiplier. A frequency of 10 MHz is merelyexemplary, but is usefully low as a synchronization signal to allowgreater lengths of transmission line runs without being so low as tointroduce an unreasonable amount of phase noise.

The IF signal generated at a mixer from the LO signal and RF signal isconverted to a digital signal by a limiter. The digital signal isprovided as input to the PLL and to the frequency multiplier thatmultiplies the signal for synchronously sampling the IF signal based onthe amount of sampling performed for measurements (e.g., M=5 for 50 MHzsampling). The IF signal is also provided as the synchronization signalto the remote receiver. The IF signal is output as the synchronizationsignal directly from a synchronization output port to the coax cable ortwisted pair transmission line.

The remote receiver uses the synchronization signal received at asynchronization input port (SYNC IN) to synchronize a LO signal and anA/D clock signal. The synchronization signal is digitized and thenprovided to a PLL and a frequency multiplier. The LO signal issynchronized using the PLL and down converts a received test signal toan IF signal that is then sampled to obtain a transmission measurement.The synchronization signal is further provided to an A/D to generate areference signal. A test signal and the reference signal can be ratioedto obtain a forward transmission (S21) measurement at the remotereceiver.

As above, in a further embodiment a system and method for synchronizinga measurement instrument and remote receiver can use fiber optic cablefor the transmission of the synchronization signal. In such anembodiment, the synchronization signal is converted from an analogsignal to an optical signal and transmitted to the remote receiverwhereupon the optical signal is converted back to an analog signal foruse in synchronizing the LO and A/D clock signal of the remote receiver.The remote receiver likewise relies on a PLL to synchronize the LO andA/D clock, but generates a digital signal from the analogsynchronization signal.

FIG. 4 is a circuit diagram of the PLL used in FIGS. 2 and 3 forphase-locking a LO signal to an RF signal, and to a digitalsynchronization signal, in accordance with an embodiment. The circuit isisolated to highlight the generation of the synchronization signal. Asshown, the LO has a frequency F_(LO) greater than the frequency F_(RF)of the RF signal, and an IF signal having frequency F_(IF) generated bya mixer as the difference in frequency of the LO and RF signals(F_(IF)=F_(LO)−F_(RF)). A digital synchronization signal is generatedfrom the IF signal by a limiter and has a frequency F_(SYNC) equal tothe frequency of the IF signal (F_(IF)=F_(LO)). The synchronizationsignal is provided to the PLL and becomes a reference for the LO and thePLL drives the frequency of the LO until the frequency divided by someinteger (N) matches the synchronization frequency (i.e.,F_(LO)=N×F_(SYNC)). The LO is thus locking to its output to track thephase of the RF signal. FIG. 4 further includes expressions and formulaeindicating that N program (N_(p)) must be an integer, and oncedetermined, F_(IF) and F_(LO) can be determined exactly.

In the embodiments shown in FIGS. 2-8, the target frequency of the IFand synchronization signals is 10 MHz; however; in other embodimentssome other target frequency can be used. The frequencies, integers,sampling rates, and other values used herein are merely exemplary, butrepresent possible design choices.

FIG. 5 is a circuit diagram of a VNA and a remote receiver connectedthrough a DUT and synchronized using a single, low frequency digitalsignal that is transmitted from the measurement instrument to the remotereceiver, in accordance with an embodiment. The VNA and remote receiverform a two-port network for measuring S-parameters. A LO signal and anA/D clock signal are phase-locked by a PLL as shown in FIG. 4 anddescribed above for use in making S-parameter measurements. The LOsignal and the A/D clock signal are synchronized by IF signals fromwhich a digital synchronization signal is generated by a limiter. The LOsignal is used to downconvert incident (INC) and reflected (REFL) RFsignals to IF signals for measuring S-parameters. The A/D clock signalis used to sample the downconverted IF signals. Both the LO and A/Dclock signals are synchronized to a reference signal (REF) generated bya reference oscillator. An RF source generates test signals fortransmission to the DUT and is also synchronized to the referencesignal.

As shown, incident power waves (INC) are coupled by the VNA to a mixerfor downconversion and then sampled to obtain an “a1” measurement.Likewise, reflected power waves (REFL) are coupled by the VNA to anothermixer for downconversion and then sampled to obtain a “b1” measurement.From the a1 and b1 measurements, the magnitude and phase of the forwardreflection (S11) parameter can be determined. Transmitted power waves(TRANS) are coupled by the remote receiver to a mixer for downconversionand then sampled to obtain a “b2” measurement. The b2 measurement can becommunicated to the VNA via data transmission for use by the VNA incalculating a forward transmission (S21) parameter. The remote receiveris synchronized to the VNA by the synchronization signal received by theremote receiver to phase-lock the LO and A/D clock signals of the remotereceiver via a PLL. These signals retain the phase component of thereceived test signal to the reference signal at the source.

FIG. 6 is a circuit diagram of a measurement instrument and a remotereceiver wherein a length of a transmission path of a synchronizationsignal can be determined by the measurement instrument, in accordancewith an embodiment. A synchronization signal is transmitted by themeasurement instrument to the remote receiver using a single length offiber optic cable. The fiber optic cable is connected at the measurementinstrument and the remote receiver by a duplexer that transmits andreceives the synchronization signal at different wavelengths (e.g., 1310nm and 1550 nm) so that the measurement instrument can transmit andreceive signals at the same time.

The measurement instrument comprises a coarse counter and a fine counterfor determining the length of the transmission path based on detectionof a runt inserted into a synchronization signal transmitted from themeasurement instrument to the remote receiver and retransmitted from theremote receiver back to the measurement instrument. The coarse counterincludes a divider that divides the frequency of the synchronizationsignal to synchronize the coarse counter with the synchronization signaland provide a reset signal (RST) that spans a number of cycles of thesynchronization signal corresponding to the integer number of thedivision.

As shown, the coarse counter includes a divider that divides thefrequency of the synchronization signal by 1000 to generate a signalused as a coarse counter reset over a wavelength of 100 microsecond (μs)with an integer number of cycles of the synchronization signaltransmitting over the wavelength being 1000. This provides a coarsecounter ranging from 0 to 999 cycles. Over a fiber optic cable a maximumlength of approximately 15 kilometers (km) is traversed in bothdirections over a count of 1000 cycles (c*100 μs≈30 km), assuming apropagation velocity approximating 100%.

Referring again to FIG. 6, as shown the synchronization signal of 10 MHzcan be used to create pulses of 50 ns in length. The coarse counterincludes a runt generator that inserts a runt pulse of 25 ns in lengthinto the synchronization signal. The synchronization signal with therunt is transmitted to the remote receiver, which retransmits thesynchronization signal back to the transmitter. A runt detector detectsthe runt and sends a signal to the counter to mark the count. If therunt is generated when the count is reset, the count will indicate thenumber of cycles that have passed before detection of the runt fromwhich the length of the transmission line and the number of cycles ofphase (N_(c)) is determined. The coarse counter is reset by a signal(RST) from the divider.

The run detector comprises a runt generator that produces a runt of 37.5ns in length for each pulse detected in the synchronization signalreceived at the measurement instrument and provides the runt to a pairof D flip flops. As shown in the signal box for the runt detector, whenthe value D of the first flip-flop is below the value of the 37.5 nsrunt, the flip-flop outputs a Q of 1, which sends a load (LD) signal tothe coarse counter indicating the detection of the runt. The coarsedistance measurement (DIST_(c)) and the coarse phase measurement (Φ_(c))can be calculated using the formulas:DIST_(c)=15 m*NcΦ_(c)=360*Nc

The coarse loop has a 30 m delay resolution based on the period of thesynchronization signal of 100 ns. The fine loop allows the measurementinstrument to resolve the resolution to provide accurate distance byinterpreting between the period of the synchronization signal. The fineloop comprises a phase-to-voltage converter including a pair of D flipflops. The first flip-flop receives the transmitted synchronizationsignal (A) and a signal from the second flip-flop and outputs a voltage.The second flip-flop receives the received synchronization signal (B)and outputs a value to the first flip-flop. The output of the flip-flops(C) is a voltage (V_(F)) that, when ratioed over a reference voltage(V_(REF)) of a phase detector provides a percentage of the cyclesynchronization signal. The fine distance measurement (DIST_(F)) canthen be calculated based on the length of the period dividing in halffor the two-way trip and multiplied by the ratio of voltages. The finedistance measurement (DIST_(F)) and the coarse phase measurement (Φ_(F))can be calculated using the formulas:DIST_(F)=15 m*(V _(F) /V _(REF))Φ_(F)=360*(V _(F) /V _(REF))

The total distance and total phase are therefore calculated using theformulas:DIST=15 m(Nc+(V _(F) /V _(REF)))Φ=360(Nc+(V _(F) /V _(REF)))

FIG. 7 is a circuit diagram illustrating an implementation of themeasurement instrument of FIG. 6, in accordance with an embodiment. Thecounter can comprise, for example, a series of flip flops sampling thecounter's value. A saw-tooth voltage steps higher and higher until therunt detector sends a signal to a latch. The circuit also uses a phasedetector to output a voltage (Vo) from which a portion of the period ofthe synchronization signal is determined.

FIG. 8 is a circuit diagram of a measurement instrument and a remotereceiver connected through a DUT and phase-locked to match positiveedges of a synchronization signal at the output of the measurementinstrument and at the input of the remote receiver, in accordance withan embodiment. The phase of a transmitted signal down a fiber optic linkto a remote receiver is adjusted by returning the transmitted signal tothe point of origin. The phase adjustment at the transmitting point isset to one half the phase difference between the transmitted andreceived phase. This causes the remote phase to be exactly the same asthe transmitted phase. The phase adjustment is done in a closed loopprocess in real time. The loop settling time is approximately 3 timesthe total transit time of the transmitted to receive time duration. Fora 15 km fiber length the settling time is approximately 450 μs.

The loop of the measurement instrument comprises a voltage controlledphase shifter (VCPS) to match the phase of the synchronization signal (Fangle 0) output by the measurement instrument to the phase of thesynchronization signal input of the remote receiver. A phase-to-voltageconverter drives the VCPS. The synchronization signal before and throughthe VCPS are provided as inputs to a first phase detector and thesynchronization signal through the VCPS and the synchronization signalretransmitted by the receiver are provided as input to a second phasedetector. The output of the first phase detector is inverted and theoutput of the second phase detector is halved to account for the two-waytransmission, and both outputs are then provided to a comparator. Theoutput of the comparator drives the VCPS, forcing the pulse edges of thesynchronization signal to be aligned at the measurement instrument andthe remote receiver.

FIG. 9 illustrates an exemplary single fiber, two wavelength integratedoptical transceiver for use with embodiments of the present invention.The optical transceiver relies on optical signals having two wavelengthstraveling in opposite directions. Wavelength division multiplexer (WDM)couplers integrated into the transceiver combine or separate the twowavelengths at the edges.

FIGS. 10-12 illustrate use cases for measurement instruments for andmethods of obtaining phase measurements, in accordance with embodimentsof the present invention. FIG. 10 illustrates a pair of commerciallyavailable VNAs separated by some distance. The coax can be daisy chainedat either end for more ports. The A/D clock signals of the first VNA areconnected via coaxial cable with a fiber extender (e.g., an SDR PCIxhost card) and transmitted to a fiber extender connected with the secondVNA.

FIG. 11 illustrates a use case of a ship. Typically, a ship uses cablesfed through bulkheads of the ship, for example to connect a transmissionroom with an antenna at opposite ends of the ship. Performance of thecables can be measured end-to-end by running fiber optic cable between atransmitting measurement instrument and a receiver. For example, thefiber optic cable can be connected across flight decks, down hatches,etc. The characteristics of the cables through the ship can be measured,including characteristics related to phase.

FIG. 12 illustrates a uses case of an antenna array. For example, givenone antenna common to and distant from an array of eight antennas, atechnician may want to measure the transmission between the one distant,common one antenna and the eight antennas that are separated bydifferent distances to determine any problems with the antenna paths.The antennas will all receive the same signal swept from the lowest endto the highest end of the bandwidth of the attached antenna to determinewhether the measured characteristics are the same. Because the source isone, common antenna, if the characteristics of one antenna differs, theresult could be an indication of a bad connection or loose cable, forexample.

In some embodiments, the present invention includes a computer programproduct which is a storage medium or computer readable medium (media)having instructions stored thereon/in which can be used to program acomputer to perform any of the processes of the present invention. Thestorage medium can include, but is not limited to, any type of diskincluding floppy disks, optical discs, DVD, CD-ROMs, microdrive, andmagneto-optical disks, ROMs, RAMS, EPROMs, EEPROMs, DRAMs, VRAMs, flashmemory devices, magnetic or optical cards, nanosystems (includingmolecular memory ICs), or any type of media or device suitable forstoring instructions and/or data.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the embodiments ofthe present invention. While the invention has been particularly shownand described with reference to preferred embodiments thereof, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the spirit andscope of the invention.

The invention claimed is:
 1. A measurement instrument for measuringelectrical characteristics of a device under test (DUT), comprising: asynchronization signal generator for generating a synchronization signaltransmittable to a receiver via a duplexer configured to transmit thesynchronization signal from the measurement instrument to the receivervia a fiber optic cable and retransmit the received synchronizationsignal from the receiver to the measurement instrument via the fiberoptic cable; a voltage controlled phase shifter (VCPS) connected withthe synchronization signal generator and a phase-to-voltage converterconfigured to drive the VCPS; wherein the phase-to-voltage converterreceives as inputs the synchronization signal input to the VCPS, thesynchronization signal output from the VCPS and the retransmittedsynchronization signal received at the measurement instrument from thereceiver, and wherein an output of the phase-to-voltage converter isprovided as input to the VCPS.
 2. A system for measuring electricalcharacteristics of a device under test (DUT), comprising: a measurementinstrument adapted to be connected with the DUT for transmitting testssignals to the DUT, the measurement instrument comprising asynchronization signal generator; a receiver adapted to be connectedwith the DUT and arranged remote from the measurement instrument toreceive the test signals transmitted by the measurement instrument tothe DUT; a duplexer configured to transmit a synchronization signal fromthe measurement instrument to the receiver and retransmit the receivedsynchronization signal from the receiver to the measurement instrument;wherein the measurement instrument comprises a voltage controlled phaseshifter (VCPS) connected with the synchronization signal generator and aphase-to-voltage converter configured to drive the VCPS; wherein thephase-to-voltage converter receives as inputs the synchronization signalinput to the VCPS, the synchronization signal output from the VCPS andthe retransmitted synchronization signal received at the measurementinstrument from the receiver, and wherein an output of thephase-to-voltage converter is provided as input to the VCPS.
 3. A methodfor measuring electrical characteristics of a device under test (DUT)based on transmitted test signals, comprising: using a measurementinstrument comprising a synchronization signal generator, a voltagecontrolled phase shifter (VCPS) connected with the synchronizationsignal generator and a phase-to-voltage converter configured to drivethe VCPS; using a receiver; wherein the phase-to-voltage converterreceives as inputs the synchronization signal input to the VCPS, thesynchronization signal output from the VCPS and the retransmittedsynchronization signal received at the measurement instrument from thereceiver, and wherein an output of the phase-to-voltage converter isprovided as input to the VCPS, connecting a test port of the measurementinstrument to the DUT; connecting a test port of the receiver to the DUTsuch that the receiver receives test signals transmitted by themeasurement instrument and through the DUT; connecting a synchronizationport of the measurement instrument to a synchronization port of thereceiver via a fiber optic cable and a duplexer such that the receiverreceives the synchronization signal; and synchronizing phase of thesynchronization signal at the synchronization port of the measurementinstrument to phase of the synchronization signal at the synchronizationport of the receiver; measuring electrical characteristics of the DUTbased on the transmitted test signals.